What physiologic changes take place during the transition from intrauterine to extrauterine life?
The cardiopulmonary systems undergo a rapid change from fetal to extrauterine life. At birth the umbilical cord is clamped, and systemic vascular resistance rises. With the newborn’s first breaths (increasing the neonate’s Pa o 2 and pH), pulmonary vascular resistance decreases, thereby causing an increase in pulmonary blood flow. Blood flow through the foramen ovale and the ductus arteriosus reverses direction, and then these structures eventually close. The ductus arteriosus is usually closed functionally by 15 hours of age.
If the pulmonary vascular resistance does not fall adequately, a persistent right-to-left shunt will occur (persistent pulmonary hypertension). Inability to expand alveolar spaces can cause intrapulmonary shunting of blood (hypoxia). Disruption of fetal-maternal circulation (placenta previa, abruptio placentae) can result in acute blood loss and hypovolemia in the newly born infant.
Aronson PL, Alessandrini EA: Neonatal resuscitation. In Fleisher GR, Ludwig S, Henretig FM (eds): Textbook of Pediatric Emergency Medicine, 6th ed. Philadelphia, Lippincott Williams & Wilkins, 2010.